Recovery of minerals from earth formations has been enhanced by application of electric currents. One such method of electrotreatment was used to enhance underground leaching of metals. In this method, a preliminary electromagnetic treatment was applied to leaching muds, and then pulsed currents were passed through the muds during leaching. This resulted in increases in the solution concentrations of the metals being leached.
Another method of electric treatment for enhancing the recovery of minerals from earth formation has been used in the production of petroleum fluids from petroleum bearing earth formations. According to this method, high voltages, such as 150,000 Volts, were passed through productive zones of oil bearing formations to heat oil within the productive zones in order to increase oil production. Fixed values of the high voltages were applied to the productive zones, without controlling the amount of current passed through the productive formations. This often resulted in not only heating the oil which was in the productive zones, but also caused destruction of the pore spaces in the portions of the formations which were close to the wells. This type of treatment method often had unpredictable results on formation permeabilities and other filtration parameters when applied to different geologic formations, such that the permeabilities of the formations were often reduced rather than being improved. Application of this method in actual working wells was also difficult because of the requirement for localized application of the high voltages to downhole portions of the wells located near the productive formations, and the difficulty in electrically isolating the downhole regions of the wells to which the high voltages were to be applied.
Other methods for electric treatment of earth formations have included a method for passing pulsed electric currents through formations containing clays to cause electrochemical reactions in the clays, which resulted in destruction of the clays and increases in formation permeabilities. This method was limited in its field of use since it required formations which included clays. This method also provided unpredictable results, such as unwanted deterioration of formation permeabilities. Excessive treating currents were often applied, which resulted in increases in permeability which quickly dissipated shortly after treatments were discontinued, indicating that the increases in permeability were due to gas colmatation effects. After the gas bubbles caused by gas colmatation dissolved, the permeability quickly returned to pretreatment levels. The gas colmatation effect often dissipated after a few days. Large energy losses were often encountered.
Another prior art method is that disclosed in the PCT International Patent Application having WIPO Publication Number WO/92/12326, invented by Vyacheslav Selyakov, the inventor of the present application, and entitled "Method of Controlling Rock Permeability Of Near-Face Section Of Well." This method treats rock formations with pulsed current to increase the permeability. A critical value of a treatment current density (j*) was preliminarily established by mathematical calculations for each type of earth formation. The critical value of current density (j*) was defined as a threshold level at which irreversible changes in formation permeability began to occur, as opposed to reversible increases in permeability which decreased over relatively short periods of time. Reversible increases in permeability tended to occur when the current density (j) applied to formations was less than the critical value (j*). Irreversible decreases in formation permeability were encountered when the current density (j) applied to the formations exceeded the critical value (j*). The critical value of the current density (j*) was determined by mathematical calculations which attempted to take into account the structure of the formation, the ultimate strength of the grouting materials within formation pore spaces, and the parameters of liquids within the formation.
The calculation of the critical value of the current density (j*) for this prior art method was based on providing a desired energy density for release in thin micro-capillaries of the formation. Increases in the permeability were attained due to mechanical destruction of cementing agents disposed in the thin micro-capillaries which limited the rate of filtration of fluids through the formations. By localizing the density of energy released in these micro-capillaries, destruction of the cementing agents located therein was achieved without destroying the medium as a whole. The energy released in micro-capillaries was localized due to the nonuniformity of pore spaces within the formations. For example, when an electric current is passed through two capillaries which are connected in-series, with respective ones of the capillaries having radii r.sub.1 and r.sub.2, the ratio of current densities passed through each of the two capillaries is j.sub.1 /j.sub.2, which is proportional to (r.sub.2 /r.sub.1).sup.2, and the densities of electric emission are E.sub.1 /E.sub.2, which is proportional to (r.sub.2 /r.sub.1).sup.4. In actual downhole earth formations, the ratio of the radii of adjacent capillary spaces (r.sub.2 /r.sub.1) may reach 1000 and more, resulting in large values for the density of relative energy released in smaller capillaries. The localization of energy released in the small capillaries due to the varying structure of different void spaces, which interconnect to form capillaries and provide permeable paths through rock formations, were taken into account in the mathematical calculations for determining critical treatment currents in the smaller capillaries for different types of formation rocks to attain desirable changes in permeability.
After a value was calculated for the critical current density (j*), pulse durations were chosen so as not to exceed the typical time (.tau.) for dissipation of the thermal energy in proximity to interconnected pore space capillaries located in the formation. A pulse repetition rate was selected which was less than six during a time interval which is less than the typical time (.tau.*) beyond which the gas colmatation of the rock takes place. An appreciable rise in the efficiency of electric treatment was achieved by using pulsed current with a duration not more than the distinctive time of dissipation of thermal energy in the nonuniformities of the medium having the size of about one grain radius. This made it possible to dramatically reduce the losses of energy introduced into the voids of the medium, and made the treatment of deep wells feasible. In some cases, gas colmatation resulted in temporary increases in permeability which lasted for only a few days. Some of the changes in the permeability occurred as a result of electrocapillary effects causing changes in the phase equilibrium of multiphase systems (water-oil, water-oil-gas). As a result of these electrocapillary effects, oil penetrated into the capillaries which previously contained water, resulting in decreases in water phase permeability. The decreases in water phase permeability changed the structure of filtration flows significantly, and often lasted for several months.
The above method for applying pulsed electric currents to restructure voids in formations and increase formation permeabilities proved unpredictable and problematic in that excessive currents were often applied. In order to destroy the cementing agents it was necessary that the current density in the capillaries exceed the threshold of the critical value of the current density value (j*), at which destruction of the cementing agents occurred. When the current density (j) passing through a capillary was below the threshold value, represented by the critical current density (j*), irreversible changes in permeability did not occur. Instead only reversible changes in permeability occurred, which were associated with the breakdown of films of bound liquids adhering to the surfaces of individual groups of capillaries. When the current density (j) applied to a capillary was higher than the threshold level, represented by the critical current density (j*), a gas phase would often develop, causing declines in permeability to the point of full termination of formation permeability. The changes in permeability which resulted from excessive currents being applied were often of a reversible nature, and lasted for only a few days. The excessive currents also caused gas colmatation which resulted in excessive pressures that irreversibly destroyed formation permeabilities. In some formations, excessive pressures which were above levels for dissipation of clays and cementations materials often caused damage to the formation rock matrix, which effectively destroyed formation permeability.